Roofs for buildings have many requirements. They must be waterproof to repel rain, snow, etc., they must be capable of supporting an external load, such as accumulated snow, and in most geographic areas they must also provide insulation either to prevent heat loss in winter climates, or to prevent ingress of heat into air conditioned buildings. In order to support sufficient loads it has been necessary to provide rather extensive framing. Built-up outer roof structures involving asphalt and a fabric are common in flat roofs, while slate surfaced asphaltic shingles are commonly used on sloping roofs. It is common practice to provide insulation beneath a roof. Conventional roof structures tend to be rather labor intensive, and hence expensive. Besides being expensive, labor intensive roofs tend to vary somewhat in quality in accordance with the skill of the workmen installing the roof.
There have been roof constructions made of laminated material including an insulator such as a foam plastic which have been less labor intensive than roofs that are built on the site from separate materials. However, such roofs have not always been fully satisfactory.